1. Field of the Invention
The invention relates to a drive control for a three phase AC motor via an inverter using safe technology and having a possibility for safe stopping and braking by armature short-circuiting.
2. Description of the Related Art
When using electric drives in industrial automation technology, for example in the case of numerically controlled machine tools and robots, a primary objective is the protection for both human operator and the equipment. The objective of xe2x80x9csafe stoppingxe2x80x9d function for the motor is to ensure that even in the case of a single fault, the electric machine and/or the motor cannot execute any dangerous movements. This function is selected as a rule, for example, before opening a protective door.
In one implementation of xe2x80x9csafexe2x80x9d technology, a two-fold power shutdown and resulting disconnection from the motor is used. It is generally accepted in this case, the lower and/or the upper transistors or other current valves of an inverter with the bridge circuit are to be turned off separately.
A known possibility for implementing the function of xe2x80x9csafe stoppingxe2x80x9d consists in xe2x80x9csafelyxe2x80x9d blocking the drive signals for the power transistors, which is referred to as xe2x80x9cpulse inhibition.xe2x80x9d
Here, the expression xe2x80x9csafexe2x80x9d is intended to express that the respective requirements of any applicable professional associations and/or professional institutes for safety at work are being met.
The drive is to be shut down in the best possible way in the event of a fault. The most unfavorable case is obtained in this case when a measure for pulse inhibition is performed when the drive was moving. The drive then coasts to a standstill. A particularly great danger therefore exists in this phase for operating staff, because the movements can no longer be controlled.
It is therefore desirable to accelerate the braking operation by means of additional measures and additional means. Examples of this are braking by generator (though this is not possible in the case of mains failure or system crash) or mechanical holding braking/service braking. Also known is a method of braking by what is termed xe2x80x9cbraking by armature short-circuiting.xe2x80x9d
In the case of braking by armature short-circuiting, the stator winding of the motor is short circuited via switches/contacts or power semiconductors. Kinetic energy which is present, such as is present during the above mentioned coasting of the motor to a standstill is then converted into heat via the stator winding resistors. A variant of such braking by armature short-circuiting is known from European patent application EP 0742637.
In order to effect emergency braking in the event of a fault with electric drives, it is possible to produce a controlled integrated armature short-circuiting by blocking one converter bridge using safe technology while the other inverter bridge uses clocked driving to bring about short-circuiting of the phases of the electric drive. Reaction times and braking time can be optimized by specific timing, for example by optimum torque control via a characteristic.
With regard to the highest possible safety, it would be desirable to combine the function of xe2x80x9csafe stoppingxe2x80x9d with the function of xe2x80x9cbraking by armature short-circuiting.xe2x80x9d However, this has presented substantial problems from a technical point of view, because mutually contradictory measures are involved. Whereas the function of xe2x80x9csafe stoppingxe2x80x9d requires precisely reliable blocking of the current valves, for example power transistors, the function of xe2x80x9cbraking by armature short-circuitingxe2x80x9d necessitates precisely switching through the power transistors of a bridge arm in order to produce short-circuiting of the stator winding. No solution to this problem has yet become known.
It is therefore an object of the present invention to implement the two contrasting functions in a combined fashion with one another.
In accordance with the present invention, this is achieved with a drive control for a three-phase AC motor via an inverter. The inverter includes several current valves in a bridge circuit forming an upper and lower bridge arms. The inverter also includes a first means for the pulse inhibition for the upper bridge arm of current valves, and a second means for the pulse inhibition of the lower bridge arm of current valves. The respective current valves may be selectively blocked by using the first and second means for pulse inhibition, or, in the event that the first and second means fail, by interrupting respective supply voltages for driving the current valves of the upper and lower bridge arms. The inverter also has a means for braking by armature short-circuiting, which may be used to selectively, or in the event of a fault, short-circuit the stator winding of a three-phase AC motor. This is accomplished by switching on all current valves of a bridge arm. In such a case, a logic unit may be used to provide a voltage for switching on these current valves as soon as a respective supply voltage for driving the current valves of this bridge arm and/or those of the other bridge arm are interrupted by a means for pulse inhibition.
Safety may be further enhanced when, in the event of a fault, all drive signals of a drive unit of the drive control for the current valves can be blocked by using at least one means for pulse inhibition.
It is also possible in the case of this implementation for the upper and the lower bridge arms to be blocked separately.
In one embodiment of the invention each means for pulse inhibition can supply a respective signal for driving the logic unit which is configured in such a way that in the event of a fault the voltage for switching off all current valves of a bridge arm can be generated from these signals. In the event of a fault, the logic unit can provide independently of the signals of a drive unit of the drive control a drive signal for the current valves of the bridge arm serving for armature short-circuiting.
Safety may be further improved, by a positive dynamic activation of the two pulse inhibition paths. The positive dynamic activation may be accomplished by reading the supply voltages for driving the current valves of both the first and second means for pulse inhibition of the respective upper and lower bridge arms, and activating the first and second means for pulse inhibition and a means for braking by armature short-circuiting if at least one of the supply voltages readings is lacking.
Yet another embodiment of the invention includes respective optocouplers transmitting drive signals to the current valves. In the event of a fault, the supply voltage of the optocouplers associated with the respectively assigned bridge arm can be interrupted by each means for pulse inhibition.
Safety is further increased when there is a crosswise comparison of external tripping signals assigned to the respective means for pulse inhibition. This can be performed when the two means for pulse inhibition are interconnected via a bi-directional communications interface. In the event of deviating tripping signals, both means for pulse inhibition can be tripped.
According to an embodiment of the invention, in order to drive the two signals the logic unit combines them logically with the drive signals for the current valves of the bridge arm serving the purpose of armature short-circuiting in such a way that with activation of the voltage for switching on all current valves of a bridge arm there is also activation of the drive signals for the current valves of the bridge arm serving the purpose of armature short-circuiting.
If there is a fault in the case when the two signals for driving the logic unit normally conduct high level and a voltage for switching on all current valves of a bridge arm is suppressed by appropriate inverting driver stages of the logic unit, both signals change to low level so that the inverting driver stages of the logic unit provide on the output side a voltage for switching on all current valves of a bridge arm.
A very effective and particularly cost effective logic unit may switch each current valve by a drive unit of the drive control having an associated drive signal with a logic low level. The logic unit then combines the two signals to drive the current valves via a logic AND gate. The logic signal can be further combined via respective further AND gates with the respective drive signals for the current valves of the bridge arm serving the purpose of armature short-circuiting.
Further details and advantages of the invention will become evident from the following description of an advantageous exemplary embodiment and further in conjunction with the further figures. In this case, elements having the same functionality are annotated by the same reference symbols.